A spark plug includes a center electrode and a grounding electrode. The center electrode is held by an insulator, and the grounding electrode is fixed to a metal shell which accommodates the insulator. A spark gap, which is a gap for generating spark discharge, is formed between the center electrode and the grounding electrode. The spark plug generates the spark discharge in the spark gap, and thus, ignites gas supplied into a combustion engine of an internal combustion engine.
Like the spark plug, a plasma jet ignition plug is known (for example, refer to JP-A-2009-224345). In the plasma jet ignition plug, the grounding electrode is joined to an inner circumferential surface of the metal shell and is integrated with the metal shell, the spark gap between the center electrode and the grounding electrode is surrounded by the insulator, and a discharge space having a small volume, referred to as a cavity, is formed.
In the plasma jet ignition plug, as described above, the grounding electrode is joined to the inner wall surface of the metal shell. In the plasma jet ignition plug, the joining property of the grounding electrode where it is secured to the metal shell, preferably, at a high level.
In the technology of JP-A-2009-224345, a tip portion of the insulator is strongly pressed to the grounding electrode, and thus, an object thereof is to prevent the insulator from being damaged. Accordingly, in JP-A-2009-224345, a special consideration is not made for the securing of the joining property of the grounding electrode with respect to the metal shell. Not only in the plasma jet ignition plug disclosed in JP-A-2009-224345, but also in a spark plug having a type in which a grounding electrode is welded to an inner wall surface of a metal shell as described in U.S. Pat. No. 6,064,144, there is still room for the improvement of the joining property of the grounding electrode with respect to the metal shell.
The present invention is made to solve the above-described problems, and can be realized according to the following aspects.